The invention relates to magnetic filters for the removal of macroparticles from cathodic arc plasmas.
Cathodic arc plasma deposition is a coating technology with great potential. Most importantly, cathodic arc plasmas are fully ionized and can therefore be manipulated with electric and magnetic fields. While electric fields are used to change the ion energy and thus the structure and properties of deposited films, magnetic fields are used to guide and homogenize the plasma.
However, a major obstacle to the broad application of cathodic arc plasma coating is the presence of macroparticles in the plasma, where macroparticles broadly encompasses all particles much larger than the ions, including droplets, microparticles, and nanoparticles.
Cathodic arc current is localized in minute nonstationary cathode spots. Spot formation is necessary to provide sufficient power density for plasma formation, electron emission, and current transport between the cathode and anode. Macroparticles originate from plasma-solid interaction at cathode spots.
Many approaches have been proposed and tested to eliminate macroparticles from cathodic vacuum arc plasmas. Most successful are curved magnetic filters, originally introduced by Aksenov and co-workers in the late 1970s. Although high-quality metal, metal-compound, and diamond-like carbon films have been synthesized by filtered cathodic arc deposition, macroparticle filters suffer from two major drawbacks: (1) the plasma transport is inefficient, i.e. only a fraction of the original (unfiltered) plasma is actually useable for film deposition, and (2) the removal of macroparticles is not complete. The latter is particularly pronounced for solid macroparticles as observed with cathodic arc carbon plasmas.
The design of macroparticle filters depends first and foremost on the mode of arc operation. DC arc plasma sources are usually equipped with cathodes of large size, e.g. diameter of 3-5 cm. The spot location may be magnetically controlled. In any case, the location(s) of plasma production, the micron-size cathode spot(s), can vary across the cathode surface, and the cross section of the filter entrance must be large enough to accommodate the various spot locations. A large filter entrance necessarily implies a large filter in length, volume, and weight. However, the plasma density in the filter drops exponentially with the path length of the filter.
Most filters, and virtually all of the DC-operated filters, have a xe2x80x9cclosedxe2x80x9d architecture in the sense that the filter volume is enclosed by a tube or duct which is surrounded by magnetic field coils. Macroparticles cannot leave the filter volume. They are expected to stick to the duct wall or to be caught between baffles that are placed inside the duct. The ducts are preferably bent, e.g. at 45xc2x0 or 90xc2x0, so there is no line-of-sight from the arc spot to the substrate.
U.S. Pat. No. 6,031,239 shows a filtered cathodic arc source with a filter of closed architecture having a toroidal duct with two bends, preferably in different planes, and a liner or baffle in the duct. The double bend provides no line-of-sight and no single bounce path through the duct. The duct is relatively large, with a diameter of 4-6 inches.
However, catching macroparticles is difficult for some cathode materials such as carbon because the macroparticles tend to be elastically reflected from surfaces. This xe2x80x9cbouncingxe2x80x9d problem is addressed by filters with open architecture where xe2x80x9cbouncingxe2x80x9d is used to let macroparticles escape from the region of plasma transport. Filters of open architecture do not have a duct but consist of a few turns of a magnetic field coil. The coil must have a relatively high current to generate sufficient field strength despite the small number of turns per length. For convenience, the arc current can be used in the filter coil.
Thus a short, open-architecture magnetic filter in combination with a compact arc source with a cathode of small area and operated in pulsed mode is desirable in order to have a high throughput of clean plasma to a deposition target.
Accordingly it is an object of the invention to provide an improved macroparticle filter for cathodic arc plasma sources.
It is another object of the invention to provide a magnetic filter for use with a compact arc source with a cathode of small area and operated in pulsed mode.
It is also an object of the invention to provide a short, open architecture magnetic filter for the removal of macroparticles from cathodic arc plasmas.
It is a further object of the invention to provide a filter suitable for cathodic arc sources for the deposition of thin amorphous carbon films.
The invention is an open-architecture, bent solenoid magnetic filter, with additional field coils at the filter entrance and exit, which improves macroparticle filtering for cathodic arc sources. In particular, a three-dimensional double-bent filter formed of an S-shaped coil with the two bent sections twisted out of plane forms a very compact and efficient filter (twist filter). The coil turns further have a flat cross-section to promote macroparticle reflection out of the filter volume. Thus the coil itself serves as a baffle structure. The filter turns are held at numerous points for cooling and stability reasons; despite the relatively short pulse length of 1 ms or less, there is a significant electromagnetic force on the filter turns when the current exceeds 1 kA. The twist filter has a strong magnetic field ( greater than 100 mT), providing excellent plasma confinement. The fringe field at the entrance is well suited for source-filter coupling. The twist filter is particularly suitable for a miniaturized, pulsed cathodic arc plasma system for the deposition of ultrathin amorphous hard carbon (a-C) films, e.g. for the magnetic storage industry.
Further in accordance with the invention, an optional output conditioning system for expanding the plasma beam to cover a larger area of the target may be implemented at the output of the magnetic filter. The expander system has three basic components: an expander coil, followed by a straightener coil, followed by a homogenizer.